William Burkitt on Revelation 22:21

This is an epistolary conclusion, used almost in all the epistles of the New Testament, and so here, because this prophecy was in the manner of an epistle directed to the churches.

St. John began this epistle with this salutation, Rev 1:4 and so ends with it here; he knew there would be a long tract of time intervening between the giving of the promise and the fulfilling of it, between the bride's making herself ready, and the day wherein the marriage was to be solemnized; he well knew that before the consummation of all things, there were many sad dispensations which the churches would certainly meet with, many false doctrines would be spread, many duties to be performed, many afflictions to be endured, all which would require an extraordinary assistance and special grace; therefore he prays that the grace of our Lord Jesus Christ may be with them all; teaching us incessantly to desire for ourselves, and to crave for others the auxiliary aids of divine grace, to enable to the acceptable performance of every duty, and to the patient enduring of every trial.

The greatest mercy that can be wished to any, or that any can wish for themselves is the grace of God, to excite and quicken us to our duty, and to assist and help us in the performance of it; and accordingly St. John shuts up this book, and therewith the canon of the whole scripture, in these words, The grace of our Lord Jesus Christ be with you all. Amen.